When forming a well bore, various forms of earth boring bits are used to cut through the hard material formations in the earth. One type of drill bit utilizes one or more rolling cutters whose outer surfaces include projections such as milled teeth or cutter inserts. Each cutter is mounted to rotate about a supporting shaft or spindle extending from the drill bit. Typically, the spindle axis is spaced radially from and inclined with respect to the rotational axis of the drill. The incline of the spindle axis causes the cutter to both rotate about its rotational axis and roll relative to the borehole bottom as the bit body rotates. As the cutter rolls, the teeth gouge into and pulverize the formation material. As a result, the cutter disintegrates a concentric ring of formation material at the borehole bottom.
U.S. Pat. No. 3,389,760 discloses an early version of the foregoing type of rolling cutter. A rolling cone cutter is supported by and rotates about a load pin, which is supported at its ends by a generally U-shaped support saddle. A number of such saddle and rolling cutter arrangements may be mounted on a single bit body for drilling a large borehole. For disintegrating formation, a multiplicity of small inserts made from cemented tungsten carbide are fitted into holes drilled into each cutter body. These inserts are disposed in overlapping rows so that as the cutter is rolled over the hole bottom, the inserts cut overlapping tracks. As a result, the formation is disintegrated over the full width of a concentric swath defined by the radial length of the cutter with respect to the drill-bit axis. The cutting elements of U.S. Pat. No. 3,389,760 are disposed in a semi-random pattern on a smooth outer surface of the cutter. This pattern causes certain lateral discontinuities in the bottom hole pattern. As a result, the discontinuous succession from one cutting element to another during drill-bit rotation often imparts an abrupt impact force to the drill assembly. Furthermore, the outer surface of the cutter lacks relief grooves, which aid the initial removal via drilling fluid of a disintegrated formation.
U.S. Pat. No. 4,393,949 discloses another prior-art roller cutter, which includes a helical cutting tooth protruding from the cutter body. The helical shape of the tooth functions to cut along the full width of the concentric swath formed by the roller cutter as the bit is rotated. However, the helical path of the tooth is open, i.e., it does not close upon itself. The open-ended helical cutting structure produces a bottom hole pattern resembling a series of skewed or spiraled open-ended grooves. These grooves may subject the lead edge of the cutting tooth to an abrupt load with each revolution of the cutter about its axis.